The present invention relates to an image forming apparatus and, more particularly, to an image density control method for an image forming apparatus which controls image density by controlling an image forming condition on the basis of a plurality of exclusive pattern for control formed on a photoconductive element, or image carrier.
A prerequisite with an electrophotographic copier, facsimile machine, printer or similar image forming apparatus is that high quality images having stable density be produced over a long period of time. In an electrophotographic copier, for example, a photoconductive element is often implemented as an OPC or a selenium-based organic photoconductive element. An OPC type photoconductive element in particular has a problem that the potential disposited in a low potential area thereof (white area) is susceptible to a change in ambient temperature or the deterioration of the element and has substantial influence on the image forming conditions. In the light of this, a copier of the type described is provided with various image density control devices such as a toner density control device. Specifically, it is a common practice to form an exclusive pattern (black pattern) for controlling image forming conditions on a photoconductive element, to sense the exclusive pattern by a sensor, and to control image forming conditions on the basis of an output of the sensor. Such a pattern is formed on a photoconductive element while holding a scanner in a halt, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 59-2058 by way of example.
The image forming conditions are effected not only by the change in the potential deposited in a low potential area of a photoconductive element but also by the eccentricity of the element. Specifically, eccentricity of a photoconductive element renders the sensed value associated with the exclusive pattern unstable. To reduce the influence of such factors, a plurality of exclusive patterns for control may be formed at different positions on a photoconductive element. The above-stated prior art, however, cannot implement the multiple pattern scheme because it forms a pattern while maintaining a scanner in a halt.